Nature of the real structure of graphite-like BN and its transformation into a wurtsite modification under impact compression

Self Cite

The pattern of phase transformations in boron nitride under high-temperature shock compression has been studied using a previously proposed method for high-temperature shock-wave synthesis of highpressure phases followed by rapid quenching. Fine powders of turbostratic and partially ordered graphite-like BN were used as initial structures. Shock compression was carried out in ring devices at a pressure of 30 GPa and a temperature above 2500 K. A mixture of dense phases (wurtzitic and sphaleritic) was found to form from the graphite-like structures under those conditions; the total yield of those phases and the relative amount of the sphaleritic modification are considerably higher when turbostratic BN is the starting material. Both of the dense phases formed have a nanocrystalline grainstructure. The wurtzitic phase does not transform into the sphaleritic phase under those conditions, which points to cubic BN forming directly from the graphite-like structures.Under shock compression highly crystalline graphite-like boron nitride BN g is transformed into the wurtzitic modification BN w , which is a metastable high-pressure phase. The transition occurs in the pressure range 13-19 GPa;the temperature corresponding to the onset of the transition in pore-free specimens is 650 K. The shock compression temperature T sh rises as the pressure and initial porosity increase [1].The degree of the BN g → BN w transformation under constant p, T during the compression increases with the degree of the three-dimensional order P 3 of the initial structure. This is attributed to the martensitic mechanism of the transition [2]. After the load is removed the relative yield of the dense phase in the preserved specimen may be significantly lower than the degree of the high-pressure phase transformation because of the reverse transition under the effect of the high residual temperatures T res . In order to lower those temperatures cooling additives (e.g., water, copper powder) are introduced into the shock-compression device along with the material being compressed [1].Martensitic transformations are not possible in turbostratic (P 3 = 0) disordered graphite-like structures. At sufficiently high temperatures, however, such structures may undergo a diffusion transformation, forming a stable highpressure phase, cubic boron nitride with a sphalerite structure BN sph [3]. Under shock compression, however, an increase in T sh leads to higher residual temperatures, causing a reverse transition up to the complete disappearance of the high-pressure phase [1]. In [4,5] we proposed a new method of shock-wave synthesis of ultrahard phases, allowing that difficulty to be overcome. The essence of the method consists in using special additives which, while not reacting chemically with the material being compressed, further heat it as the load is applied and cool it rapidly as the load is removed. The additives

Section: Initial Specimens and Methods Of Investigationmentioning

confidence: 87%

Section: Transformation Under Shock Compressionmentioning

confidence: 99%

Section: Transformation Under Shock Compressionmentioning

confidence: 99%

See 1 more Smart Citation

Phase transformations of disordered structures of graphite-like boron nitride under high-temperature shock compression

Kurdyumov

Britun

Borimchuk

et al. 2004

Self Cite

“…From the (002) and (004) reflections, we determined the (d 002 ) distance, which we used to estimate the concentration of turbostratic stacking faults in BN (γ) using the γ(d 002 ) dependence from [3].…”

Section: Methodsmentioning

confidence: 99%

Dependence of the Structure of Sintered Boron Carbonitride on the Defect Level in the Starting BN Powder

Grigor'ev

Bega

Lyashenko

et al. 2005

“…It has been established [3] that the transformation occurs within the pressure range from 12 to 21 GPa, and the temperature of onset for the transformation is close to 500 K. Study of the specimens after relieving the pressure [1] showed that a wurtzite modification (BN w ) is formed as a result of the transformation. The transformation BN h → BN w is diffusionless (martensitic), and for the p,T shock compression conditions indicated above, it occurs only when the original BN h structure is highly ordered [4,5].…”

Section: Introductionmentioning

confidence: 99%

Development of a Method for High-Temperature Shock Compression of Powder Mixtures to Obtain Cubic Boron Nitride

Britun

Kurdyumov

Borimchuk

et al. 2005

Self Cite

We used shock compression of a mixture of boron nitride powder with an alkali halide salt as an additive to realize the phase transformation of turbostratic BN to the cubic modification. We have experimentally studied the effect of the type and amount of the additive and also the initial density of the mixture on the yield of the cubic phase of BN. We have roughly calculated the pressure and temperature arising upon shock compression of powder mixtures in an annular cylindrical storage ampul. We have shown that melting of the additive has a substantial effect on development of the phase transformation in boron nitride under shock loading conditions.